This expression is used in police work. It refers to a particular criminal or criminal group's habitual method or manner of carrying out a crime. The modus operandi (or MO) is one of many characteristics of a criminal used to identify him or her (or them), and to tie crimes to those who commit them.

Modus Operandi is also the name of an online text game from Simutronics, the makers of Dragonrealms and Gemstone III. Basically, it's a detective game set on the Carribean island of Morada. You run errands and chase down baddies for xp and loot. I remember playing it a bit when it was free on AOL (it was the only service available here in 1995. Sue me), and it wasn't really all that fun. You can find out more about it at http://www.play.net/ The charge is $9.95/month for basic play, and after that things get tricky, but that's another node.

The "MO," as it's abbreviated, is what the perpetrator does to commit his or her crime. It's an extremely important concept in criminal investigation. It is learned, and can change to suit the crime.

In many judicial systems, the only way to link different crimes together is through the MO. If there were a string of killings, all women, where the assailant shot from a blitz attack from behind, usually around the same type of day, then you could be led to think that the cases are connected, they have generally the same MO.

For example, there was a bank robber in Grand Rapids, Michigan. He would go in, wave his gun around, and force everyone in the bank to undress. He did it so the witnesses would be preoccupied and embarrassed, and have less chance of identifying the robber in a police lineup later on.

This was the robber's Modus Operandi, as it helped him accomplish his crime. If he went on a string of robberies, he would probably use the same tactic, it didn't fail before, so it shouldn't fail on the next bank. However, once he gets caught, there is a clear line of evidence linking all the cases, what are the odds another robber would do the same thing, and look similiar, and drive a similar getaway car?

Try not to confuse this with a criminal's signature; as it's similiar but a different concept. There was a different real-life bank robber who would make everyone in the bank undress, but then snap photos of them. It didn't help his robbery; it slowed him down and made it more likely he would get caught, but he did it for pleasure. That's signature.